[ELF] When the code segment is the last, align it to the page boundary

When the data segment is the last segment, it is correct to leave
it unaligned. However, when the code segment is the last segment,
it should be aligned to the page boundary to avoid loading the
non-segment parts of the ELF file at the end of the file.

Differential Revision: https://reviews.llvm.org/D33630

llvm-svn: 309829